Field of the Invention
Embodiments described herein relate generally to wristbands for automatic identification and tracking, and, in particular, to a multipurpose wristband apparatus that can incorporate any type or combination of types of automatic identification technology such as barcode, passive radio-frequency identification (RFID), battery-assisted passive (BAP) RFID, and/or active RFID.
Description of the Related Art
The technique of identifying objects using radio-frequency communications has been eponymously called radio-frequency identification (RFID). RFID systems have been employed in an increasingly wide range of applications such as retail supply chain, postal logistics, healthcare, manufacturing, retail stores, airport baggage tracking, hospitality, social media, travel, theme parks, etc. In retail supply chain applications, RFID has been used to track and trace goods throughout the supply chain, automate the receipt of pallets of shipments at distribution centers, increase shipping accuracy of goods from distribution centers (DCs) to stores, and manage inventory throughout the supply chain. In postal logistics RFID has been used to monitor the quality of service of postal shipments for international and national mail systems. For instance, a global postal organization has deployed RFID to over forty countries around the world (and increasing) to measure and monitor quality of service of mail delivered between those countries. In healthcare, RFID is being used for asset and resource management, as well as patient and staff tracking for improving patient flow within hospitals. In airports, specifically baggage tracking, RFID is being used as a replacement to barcode-based systems for quicker, more secure, and more accurate transfer of bags to improve the overall baggage handling rate.
Accordingly, RFID systems have been increasingly employed in diverse applications to facilitate the identification and tracking of merchandise, personnel, and other items and/or individuals that need to be reliably monitored and/or controlled within a particular environment. The introduction of RFID into these applications has resulted in more secure, efficient, and accurate systems.
A conventional RFID system typically includes at least one RFID transponder or “tag,” at least one RFID reader (or interchangeably referred to as an “interrogator”), and at least one controller or server. The reader inventories the tags and forwards the data to the server or controller.
At the physical layer of a passive ultra-high-frequency (UHF) RFID system, RFID tags communicate by “backscattering” signals that are concurrent with reader transmissions, and using a variety of frequencies and encodings under the control of the reader. This is in contrast to earlier high-frequency (HF) tags based on inductive coupling that only provided read ranges of centimeters, and active tags that require batteries to increase their range. There is a class of tags called battery-assisted-passive (BAP) that may also be of interest. For some applications, more range or link margin may be needed than a passive tag, especially in environments with metals and water in which electromagnetic waves experience shadowing of energy, destructive interference, or strong attenuation. More link margin may lead to better reading reliability and better interference control in harsh environments. BAP tags may overcome the read sensitivity limitation of passive tags by adding a battery to power the chip. The radio-frequency (RF) signal is then only used to carry the information, not to supply power to the chip. These tags retain the reverse link of passive tags, i.e., backscatter the response. BAP tags fill the gap between purely passive tags and the more costly (battery-powered) active tags.
Each RFID reader typically follows a predefined sequence or protocol to interrogate and retrieve data from one or more RFID tags within the RF field of the reader (also known as the “interrogation zone” of the reader). It is noted that the interrogation zone of a reader is generally determined by the physical positioning and orientation of the reader relative to the tags, and the setting of various parameters (e.g., the transmit power) employed by the reader during the interrogation sequence.
In systems employing passive tags, the interrogation zone is typically defined by the power coupling zone. For example, a typical interrogation sequence performed by a RFID reader includes transmitting a continuous wave (CW) to one or more passive tags within the reader's interrogation zone to power the tags, and transmitting a message packet (e.g., a request or command) by modulating the carrier signal. The passive tag then reads the message packet while tapping some of the energy of the CW to maintain its power. The message packet typically identifies one or a subset of the tags within the interrogation zone as the designated target of the message packet, and provides a request or command that the designated tag is expected to perform. After the passive tag reads the information carried by the modulated carrier signal, the tag appropriately modulates the CW, and reflects a portion of the modulated wave back to the reader by changing the reflection characteristics of its antenna via a technique known as “backscatter modulation.”
The physical and logical layers of the communication between the Reader and the tag are defined by the air protocol. Specifically, the air protocol defines the signaling layer of the communication link, the reader and tag operating procedures and commands, and the collision arbitration (also known as “singulation”) scheme to identify a specific tag in a multiple-tag environment. The world-wide standard air protocol in the UHF band is currently the EPCGlobal Class-1 Generation 2 (ISO 18000-6c) protocol (“Gen2 protocol”). Embodiments disclosed herein may use—but are not limited to using—the Gen2 protocol for communications between the reader and tags.
The collision arbitration (i.e., singulation) algorithm used in the Gen2 protocol is called the “Q algorithm” and is a variant of the slotted Aloha protocol. At the beginning of a round, the reader broadcasts the round size S to all the tags in its field of view. Each tag, upon receipt of this initial message, generates a pseudo-random number between 1 and S. That becomes the target time-slot in which the tag responds. The reader is the time-keeper and advances time by sending slot messages to the tags. Each tag decrements its target slot counter, and when the counter hits zero, the tag responds to the reader. At the reader receive side, the reader listens for a tag response in each slot. If exactly one tag responds, it initiates a state machine to transact with the tag. In the case of a collision or an “empty” slot, the reader either decides to resize S and start a new round or proceeds with the current round. This is how a single RFID reader is able to identify multiple tags in a rapid manner. For example, the singulation rate in a dense reader environment is roughly two-hundred tags per second.
The communication protocol used between the reader and the controller or server is called a reader protocol. The EPCGlobal Low Level Reader Protocol (LLRP) is currently the standard reader protocol that is employed by most conventional readers around the world. Embodiments disclosed herein may use—but are not limited to using—the LLRP protocol for communications between the reader and controller or server.
UHF RFID readers operate in the industrial, scientific, and medical (ISM) band and are prone to external interference from cordless telephones, wireless headsets, wireless data networks, etc. In addition, there may be interference due to other co-located readers. Each reader's RF receiver front end must be designed to withstand high-interference signal levels without introducing distortion that can cause tag response decoding errors. The receiver noise needs to be low so that it has sufficient dynamic range (transmit power-received tignal power from the tag) to allow error-free detection of low-level responding tag signals.
A forward-link-limited system may be limited by the receive sensitivity of the tag, and hence, beyond a certain distance, there may not be enough RF energy incident on the tag to energize it and then subsequently backscatter its response. On the other hand, a reverse-link-limited system may be limited by the receive sensitivity of the reader, and hence, beyond a certain distance between the tag and the reader, the reader may not be able to decode the tag responses correctly. Passive UHF RFID systems are typically forward-link-limited. This is because the state-of-the-art reader manufacturers have done a very good job at designing in sufficiently high dynamic range such that a reader is never backscatter-limited for passive UHF tags. The dynamic range of the state of the art UHF reader is about 120 dBm and improving. A 120 dBm dynamic range gives a RF link budget of 60 dBm each way. Thus, starting at a transmit power 30 dB with a 6 dB gain, and the forward-link budget of 60 dBm, the limiting receive signal strength at the tag is −24 dB, which is much lower than the receive sensitivity of the best tag available in the marketplace of −18 dBm. Notably, the FCC limits maximum radiated energy—the combination of transmit power at a reader port (which can be more than 30 dB to compensate for insertion loss) and antenna gain—at 4 watts EIRP (equivalent isotropically radiated power). Thus, the bottleneck with conventional UHF readers and passive tags is the forward link to the tag.
However, if a battery assisted passive (BAP) tag is used, a different result may occur. The receive sensitivity of the state of the art BAP tag is −30 dBm. This means that even at the limit (or the reader's dynamic range) and beyond it, the BAP tag can be powered and respond to the reader's signal. This means that the system becomes reverse-link-limited when interrogating BAP tags. This places stress on the design and implementation of the reader's receive path.
Certain behavior characteristics of electromagnetic fields may dominate at one distance from a radiating antenna, while a completely different behavior may dominate at another distance. At UHF frequencies, tags primarily use electromagnetic coupling in the far field, which means that the readers couple with the tags primarily with propagating electromagnetic energy in the far field (e.g., distance greater than two wavelengths). However, when the tag is in the near field (e.g., distance less than one wavelength) of the reader antenna, coupling occurs using inductive coupling. One may design tags to couple with a reader antenna primarily using inductive coupling, giving rise to UHF near-field tags. Embodiments disclosed herein may use—but are not limiting to using—UHF far-field tags.
A tag inlay may comprise a substrate, an antenna, and an integrated chip (IC). The inlay may be incorporated into a label (optionally printable) with pressure-sensitive adhesive or encapsulated in some other way.
The focus of UHF passive tags has been low cost designs. This has led to very simple antenna designs, primarily strip-line dipoles. Antennas are commonly made of aluminum, copper, silver ink, or other low-cost materials. The power transfer efficiency is the measure of the impedance mismatch between the antenna (RA+jZA) and the IC(Rchip+jZchip) and is given by τ=(4R↓chipR↓A)/(|Z↓chip+Z↓A|)↑2. Antennas are typically designed to maximize the power delivered to the IC, and this typically happens only if antenna impedance is the complex conjugate of the IC impedance (also referred to as “impedance matching”).
Conventional tag designs are typically passive RFID tags, meant for general purpose supply-chain use cases, specifically designed for free space. The performance of such tags may degrade when placed near high dielectrics such as water. The dielectric constant of water is eighty. This loss of performance may result because the close proximity to high dielectric material may cause a substantial shift in resonant frequency of the antenna causing it to not operate at a resonant mode, hence losing antenna efficiency and also causing a shift in antenna impedance which may negatively impact the power transfer efficiency.
The human body is 60% water. Thus, when a tag that is optimized for free space is applied to the human body, the read distances may be severely impacted. For instance, a tag that reads close to six meters in free space may not be readable at distances more than half a meter. Such degraded performance is typically unacceptable for a UHF-RFID-based people-tracking solution at an enterprise scale. This performance is basically equivalent to a proximity HF-based solution which is typically suitable for door-access type applications, but not for general-purpose people tracking in indoor environments (e.g., within buildings) or outdoor environments (e.g., in theme parks, ski areas, etc.). One such application of people tracking in indoor environments is patient tracking in hospitals. Patient tracking typically requires RFID tags to be in a wristband form-factor. One such application for outdoor environments is tracking skiers at a ski resort. The wristband may serve as an access pass or ticket for entry into the ski resort and/or for utilizing or accessing the ski lift or other services available at the resort, so that skiers do not have to remove their gloves in order to present the pass/ticket at an access point.
Conventional wristbands for patient identification are typically either barcode or HF tag based. Both of these technologies may allow for proximity and line-of-sight based reading. However, such limitations may not allow for patient tracking across a hospital. As mentioned above, wristband designs based on UHF passive tags may have severely degraded performance when applied on a patient's wrist.
On the other end of the spectrum, there are wristband designs based on active tags. However, wristbands built using active tags are typically bulky. They also may be very expensive (e.g., at least ten times that of UHF tag based solutions). Due to their high cost, customers conventionally reuse these wristbands. This may introduce a new workflow for the customer to manage with respect to safety, cleanliness, identity management, and battery life management.